It doesn't actually require that, plenty of ips systems can do their job
with a tap and another port for injection.

I personally don't refer to that kind of a device as an IPS. I refer to that as a "reactionary IDS". For instance, if the goal is to send a RST packet back to the SRC IP that caused the IDS to alert, then the RST packet has to beat the /actual/ ACK packet from the true DST IP back to the machine. This is essentially, for lack of a better term, a "race". This does not control traffic. Plus, it gives away the hop location of your IDS within the network to the attacker. I think if you are going to try and control traffic the much preferred method of doing so is an IPS. Traffic goes in one port, and it exits the other port. While the traffic is inside the machine, the IPS makes the decision if the traffic should exist the other port, or it shouldn't. That's a more controlling machine, thusly an IPS.

the fact of the matter is if the ids can't keep up with the presented
load that's going to be a problem whether it's inline or presented
through a tap, in the later case however it's not going to cause an outage.

True points there. However, if you purchase an IPS that is correctly spec'ed for your network (i.e. not putting a 1Gig IPS on a 2Gig link) you should have little problem being able to handle the traffic. It's mostly a software/hardware problem.

If you get a big enough box that is appropriately sized to handle the traffic, you should be able to perform the IPS function properly. Although I've seen wide variances in this. I've seen a 2Gig box do 4Gig/second of traffic, and I've seen a 10Gig box do 2 Gig/second of traffic. Test.

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